The purpose of a subscriber line interface circuit is, inter alia, to supply a two-wire telephone line with DC voltage and to transmit AC signals on the telephone line. The DC voltage shall drive a telephone or other equipment connected to the other end of the two-wire circuit, while the AC signals can be, for example, speech, number information, and private rate measurement (PRM) signals, the latter being high-frequency signal bursts that can appear at any time during a call.
The subscriber line interface circuits are so designed that a certain portion of the available battery voltage is intended for the generation of a predetermined line feed characteristic, while another portion of the battery voltage is intended for the transmission of AC signals. This latter portion is called signal headroom. Thus, the signal headroom puts a limit on the amplitude of the AC signals to be transmitted.
The PRM signals mentioned above are high-amplitude signals. Also, other high-amplitude signals may appear. To accomodate such signals, the known solutions have been either to have a constant, high signal headroom or to increase the signal headroom when "extra" signal headroom is needed, e.g. for the PRM signals. In the first case, the line interface circuits are more or less earmarked for PRM applications since more signal headroom is, in fact, reserved than what is actually needed for normal AC transmission. In the second case, control means, external to the line interface circuit, have to be provided to change the signal headroom when necessary.